1. Field of the Invention
The present invention is directed to field of automotive airbag deployment systems and more specifically to the area of diagnostic systems for detecting faults within the airbag deployment circuit.
2. Background Information
As discussed in commonly assigned U.S. Pat. Nos. 4,359,715 and 4,366,465 (incorporated herein by reference), it is important to provide a diagnostic subsystem for an airbag deployment system that continually checks the continuity of the ignitor circuit which comprises the wiring harness, electrical connectors, various crash sensors and electrically responsive explosive igniters (squibs). The igniters are conventionally wired to associated electrical connectors that provide shorting paths around the igniters when the connectors are disconnected (unmated). Such shorting insures that voltages induced into the wiring of a disconnected igniter will not accidentally energize the igniter and cause accidental airbag deployment. The diagnostic systems of the aforementioned patents sense for shorts and alarm the vehicle operator to warn of disconnected connectors and other faults which would constitute an inoperable system.
In such diagnostic systems, each crash sensor switch is shunted by a known resistance value and the electrical igniter constitutes a known resistance value. The diagnostic systems provide low current through the igniter and crash sensor circuit and detect changes in voltage levels that appear at critical points.
In commonly assigned U.S. Pat. No. 4,893,109 (also incorporated herein by reference) an igniter resistance detector is described. In laboratory tests, it was found that some airbag deployment igniter elements may suffer chemical changes that cause resistance to change from a normally 1 ohm value to a value in excess of 4 ohms. When an igniter element's resistance value degrades to that level, there is a potential that, in the event a crash actuated deployment is required, the normal actuation voltage applied to the igniter will result in below-threshold current that will fail to cause the igniter to explode. A predetermined low energy electrical pulse is periodically applied to the igniter. The duration and value of the pulse are selected so that, when added to the constant diagnostic current through the igniter, the total current will still remain well below that threshold sufficient to trigger the igniter element. During the pulse, the voltage drop across the igniter is detected and compared with values that are predetermined as being within an acceptable range. In the event the igniter resistance has increased beyond an acceptable value, a warning is then provided to the vehicle operator so that appropriate service, such as replacement of the igniter element, can be obtained.